Influence of fibrillar cellulose on pigment coating formulation rheology

Abstract

Coating suspension rheology influences the coating performance at high speeds. The quality of the coated and printed papers is also affected by the rheology of the suspension. For good coating results, it is crucial to optimize the water retaining properties of coating colours. The observation that fibrillar cellulose (MFC) could be used in paper coating formulations as a co-binder, since it is biodegradable and has good shear thinning properties, raises a question about the process ability of coating colours which have cobinder substituted with fibrillar cellulose. In this study, fibrillar cellulose is used to substitute the standard, synthetic co-binder material, carboxymethyl cellulose thus affecting both dewatering and rheological properties of coating colours. Reactivity of the MFC fibers in coating colour formulation is strongly dependent on the fineness of the fibrillar material as well as on its consistency, pre-treatment and refining route. In the present study the consolidation of coating colour layers has been studied using rheology as the main tool. This method combines the use of standard viscoelastic and static dewatering tools, Åbo Academy Gravimetric Dewatering Device and Brookfield viscometer, with the use of MRC300 Rheometer accompanied with an Immobilization cell. The rheological behaviour of all coating colours that have fibrillar cellulose as co-binders was found to be similar. Lower water retention and faster immobilization of filter cake were accompanied with higher shear thinning behaviour. This conclusion was strengthened by the recovery measurements, which showed that the fibrillar material containing coatings have a prolonged recovery time after high shear. A linear correlation between gravimetric (static) water retention and immobilization time, on the one hand and elasticity and prolonged recovery time on the other was found for fibrillar cellulose containing coating colours, regardless of the type of fibrillar material or pigment blend